In a long haul high speed optical transmission system, especially in an optical transmission line for transmitting a signal light of multiple wavelengths through wavelength division multiplexing, it is required to compensate chromatic dispersion and, if necessary, dispersion slope. For instance, optical transmission systems for compensating dispersion and dispersion slope and compensators thereof are described in the following issued patents:
U.S. Pat. No. 6,496,615,
U.S. Pat. No. 6,490,398,
U.S. Pat. No. 6,453,093,
U.S. Pat. No. 6,445,864,
U.S. Pat. No. 6,430,347,
U.S. Pat. No. 6,363,184,
U.S. Pat. No. 6,310,993, and
U.S. Pat. No. 6,301,048.
The more an optical pulse width becomes short, the more a signal spectrum widens. Accordingly, in ultra high speed optical pulse transmission systems in which a bit rate per wavelength becomes 160 Gb/s or more, the influence by chromatic dispersion including high-order dispersion in optional fiber transmission line becomes serious.
In a WDM (wavelength division multiplexing) optical transmission system in which a bit rate per wavelength is 10 Gb/s to 40 Gb/s, dispersion management for periodically compensating accumulated dispersion and dispersion slope in an optical fiber transmission line and post dispersion compensation for compensating residual chromatic dispersion at the end of the optical transmission line are used together.
The more a bit rate becomes high, the more dispersion tolerance becomes narrow. In ultra high-speed optical pulse transmission systems in which a bit rate per wavelength is 160 Gb/s or more, it is difficult to realize proper dispersion compensation using a dispersion compensating device of a fixed amount of dispersion compensation. Furthermore, chromatic dispersion in an optical fiber varies with time due to a temperature variation etc. In recent optical networks, optical signal paths are switched according to a demand and whether a fault exists. To keep up with those operations, adaptable automatic dispersion control is indispensable.
As a conventional dispersion control method, a method has been proposed in which a part of the transmission signal light is branched and converted into electrical signal, the intensity of signal clock component is monitored, and a variable dispersion controller is negative-feedback-controlled so that the intensity becomes the maximum. Such method is described, for example, in H. Ooi, T. Takahara, G. Ishikawa, S. Wakana, Y. Kawahata, H. Isono, and N. Nitamura, “40-Gbit/s WDM automatic dispersion compensation with virtually imaged phased array (VIPA) variable dispersion compensators”, IEICE Trans. Commun. Vol. E85-B, No. 2, pp. 463-469, 2002. A configuration to automatically control a amount of dispersion compensation of a dispersion compensator on a receiving end of an optical signal is described in the U.S. Pat. No. 6,370,300.
A dispersion compensator capable of controlling its amount of dispersion compensation is described in the U.S. Pat. Nos. 6,330,383 and 6,301,048.
In an optical transmission system of an optical duo-binary modulation method, a configuration to control a dispersion value of a dispersion compensator according to intensity of a specific frequency component of an optical signal is described in the US Patent Application Publication No. 2001/46077.
A configuration to detect whether an amount of dispersion compensation in an optical fiber transmission line stays within a predetermined range and to control the amount of dispersion compensation according to the detected result is described in the US Patent Application Publication No. 2001/9467.
Methods to detect and compensate waveform deterioration due to chromatic dispersion in an optical fiber transmission line are described in the U.S. Pat. No. 5,999,289.
In a conventional configuration, a clock frequency is limited to bit rates of 40-80 Gb/s or less which can be photoelectrically converted. If its bit rate is 160 Gb/s or more, it is impossible to monitor a signal quality by directly converting an optical signal into an electric signal. In a method to monitor a quality of received signal in the electric stage and to control its chromatic dispersion, it is necessary to pulse-demultiplex (DEMUX) a signal of 160 Gb/s into tributary signals of 40 Gb/s or less. However, generally, a system to monitor quality of a signal of 10 Gb/s or more requires an expensive electronic circuit and the monitored results includes deterioration of signal quality due to the DEMUX. When the deterioration of signal quality between tributary channels is varied, it sometimes causes wrong operation of dispersion control.
Even in a single wavelength transmission, ultra high speed transmission of 160 Gb/s needs to compensate dispersion slope because expansion of signal spectrum cannot be neglected.